Incipient electrical insulation failure in a polyphase alternating current electrical machine produces a slight change in the electrical behavior of the machine. For example, at the earliest stage of insulation failure on higher-voltage machines, insulation breakdown produces small, added-energy dissipations within the machine stator structure. These dissipations are detectable but usually escape notice without the aid of sophisticated monitoring equipment at the machine's electrical terminals (e.g., equipment as disclosed herein) or elsewhere. Furthermore, the occurrence of early stage faults of this nature accelerates the insulation degradation process through causing increased machine losses and the production of higher insulation temperatures. It is therefore desirable to detect such faults before maturity into a more significant fault occurs. In this regard, it is known for example that insulation life in a machine decreases by fifty percent for every ten degree centigrade rise in machine temperature under some conditions. The detection of such faults in large polyphase machines which are embedded into capital equipment systems, motors such as are used in a steel rolling mill, the motors used in mining equipment or motors used for ship propulsion, is especially desirable in view of the cost of such equipment and the significant economic impact occurring with a surprise equipment failure.
An ultimate consequence of such evolving machine insulation failures is the occurrence of a "bolted" or low-impedance turn short. At this advanced stage of failure, several prior art fault-detection approaches possess the capability to reliably detect an anomaly. A turn short creates a consistent change in the electrical behavior of the machine across the entire spectrum of its operation, including alteration of one or more of the machine phase impedances at the fundamental frequency of its energy source. To detect a bolted turn short in a wye-connected machine, for example, one need only examine each machine phase with respect to each other phase in terms of deviations in the line-neutral voltage. A system of this type has been disclosed by M. A. Cash, T. G. Habetler, G. B. Kliman, in "Insulation Failure Prediction in Induction Machines Using Line-Neutral Voltages," Conference Record of the Annual IEEE-IAS Meeting, Oct. 1997, pp. 208-212. A bolted turn short may also be detected through impedance changes as has been described by R. Maier, in "Protection of Squirrel-Cage Induction Motor Utilizing Instantaneous Power and Phase Information," IEEE Transactions on Industry Applications, vol. 28, no. 2, March/April 1992, pp. 376-380.
Turn fault detection based on both machine current and voltage change has also been accomplished in the art as is disclosed by G. B. Kliman, W. J. Premerlani, R. A. Koegl, D. Hoeweler, in "A New Approach to On-Line Turn Fault Detection in AC Motors," Conference Record of the IEEE-IAS Annual Meeting, 1996, pp. 687-693, and also by J. Sottile, Jr., J. L. Kohler, in "An On-Line Method to Detect Incipient Failure of Turn Insulation in Random-Wound Motors," IEEE Transactions on Energy Conversion, vol. 8, no. 4, December 1993, pp. 762-768. That the combination of current and voltage change may be used for detection of bolted turn shorts in a machine has also been disclosed in the same Kliman et al., vol. 8, no. 4, December 1993, pp. 762-768 publication. Detection success depends, of course, on the severity of fault in each of these instances. Generally speaking most prior art fault detection arrangements depend on both voltage and current change detection. Each of these two papers discloses a somewhat-complex sequence component based detection arrangement, an arrangement requiring considerable signal processing capability.
A turn fault in one phase of a polyphase electrical machine changes the electrical impedance of each machine phase as a result of electrical and magnetic coupling within the machine. The effect of a fault in one phase differs between the machine phases, however; this reveals not only fault presence, but also provides information as to fault severity and locale. To actually monitor phase impedance is nevertheless a considerable task, a process often requiring digital signal processing. The effects of a turn fault are found however to be more readily observed. In many circumstances, it in fact suffices for fault detection to observe either machine phase currents or phase voltages; thus, observation of both currents and voltages is not necessary.
For example, in our U.S. Pat. No. 6,043,664 "Method and Apparatus for Turn Fault Detection in Multi-Phase AC Motors" filed in the names of Gerald Burt Kliman, Thomas Gerard Habetler and Marcus Alex Cash (herein the Kliman et al. application) filed in October 1997 by General Electric Company, Schenectady, N.Y., there is disclosed a phase-voltage-based fault detection system. This application, which is hereby incorporated by reference herein, claims the benefit of a United States provisional application, Ser. No. 60/048,904, filed Jun. 6, 1997 and employs a fault detection algorithm based on line to neutral voltage sensing in a polyphase electrical machine.
Reliable monitoring of the phase currents for turn fault detection depends upon consistency of balance in the source of machine excitation in the case of a motor machine and consistency of balance in the machine load in the case of use with a generator machine. An electronic inverter energy source can provide this consistency-of-source-balance for a motor machine. Such an electronic inverter can also provide a source of variable electrical frequency energy and thus variable operating speed for the energized motor. Of course, monitored machine phase currents also depend upon mechanical load applied to the motor; however, each phase equally reflects this variable.
The only other source of change in balance in motor phase currents is internal to the machine and moreover does not depend on its wye or delta internal configuration. Given this preferred inverter excitation, a change in machine phase current balance is attributable to either a mechanical issue, such as rotor eccentricities, or an electrical fault, e.g., a fault of the type herein considered. In the present invention use of motor current changes, the effect of rotor eccentricities is suppressed by averaging. On average, for example, rotor eccentricity will affect each phase to the same degree in an asynchronous machine. In a synchronous machine system, calibration can eliminate the effect of rotor eccentricity.
The present invention therefore concerns early detection of electrical winding faults in multiple phased alternating current electrical machines, such as motor or generator, (i.e., alternator) machines, through use of line current sensing and electrical balance change detection.